Expected Losses to be Paid - Components of R&W Development (Details)	3 Months Ended			6 Months Ended					3 Months Ended	6 Months Ended	3 Months Ended			6 Months Ended					3 Months Ended			6 Months Ended					3 Months Ended			6 Months Ended					3 Months Ended			6 Months Ended					3 Months Ended		6 Months Ended		3 Months Ended			6 Months Ended					3 Months Ended	6 Months Ended			6 Months Ended				3 Months Ended	6 Months Ended						3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended																							6 Months Ended
	Jun. 30, 2014 USD ($) risk		Jun. 30, 2013 USD ($)	Jun. 30, 2014 USD ($) risk		Jun. 30, 2013 USD ($)	Dec. 31, 2013 USD ($) risk		Jun. 30, 2014 Life Insurance [Member] USD ($)	Jun. 30, 2014 Life Insurance [Member] USD ($)	Jun. 30, 2014 Prime first lien USD ($) risk		Jun. 30, 2013 Prime first lien USD ($)	Jun. 30, 2014 Prime first lien USD ($) risk		Jun. 30, 2013 Prime first lien USD ($)	Dec. 31, 2013 Prime first lien USD ($) risk		Jun. 30, 2014 Alt-A first lien USD ($) risk		Jun. 30, 2013 Alt-A first lien USD ($)	Jun. 30, 2014 Alt-A first lien USD ($) risk		Jun. 30, 2013 Alt-A first lien USD ($)	Dec. 31, 2013 Alt-A first lien USD ($) risk		Jun. 30, 2014 Option ARM USD ($) risk		Jun. 30, 2013 Option ARM USD ($)	Jun. 30, 2014 Option ARM USD ($) risk		Jun. 30, 2013 Option ARM USD ($)	Dec. 31, 2013 Option ARM USD ($) risk		Jun. 30, 2014 First mortgage, subprime USD ($) risk		Jun. 30, 2013 First mortgage, subprime USD ($)	Jun. 30, 2014 First mortgage, subprime USD ($) risk		Jun. 30, 2013 First mortgage, subprime USD ($)	Dec. 31, 2013 First mortgage, subprime USD ($) risk		Jun. 30, 2014 Closed end second lien and HELOCs USD ($)	Jun. 30, 2013 Closed end second lien and HELOCs USD ($)	Jun. 30, 2014 Closed end second lien and HELOCs USD ($)	Jun. 30, 2013 Closed end second lien and HELOCs USD ($)	Jun. 30, 2014 HELOCs USD ($) risk		Jun. 30, 2013 HELOCs USD ($)	Jun. 30, 2014 HELOCs USD ($) risk		Jun. 30, 2013 HELOCs USD ($)	Dec. 31, 2013 HELOCs USD ($) risk		Jun. 30, 2014 Student Loan [Member] USD ($)	Jun. 30, 2014 Student Loan [Member] USD ($)	Jun. 30, 2014 Asset-backed Student Loan [Member] USD ($)	Jun. 30, 2014 Public Finance Student Loan [Member] USD ($)	Jun. 30, 2014 Trust Preferred Securities Collateralized Debt Obligations USD ($)	Jun. 30, 2014 Public Finance City of Detroit [Member] USD ($)	Jun. 30, 2014 Public Finance Stockton General Fund [Member] USD ($)	Jun. 30, 2014 Public Finance Luoisville Arena Authority [Member] USD ($)	Jun. 30, 2014 Troubled Municipal Credits [Member] USD ($)	Jun. 30, 2014 Troubled Municipal Credits [Member] USD ($)	Mar. 31, 2014 Troubled Municipal Credits [Member] USD ($)	Dec. 31, 2013 Troubled Municipal Credits [Member] USD ($)	Jun. 30, 2014 Spanish sub-sovereign debt [Member] EUR (€)	Jun. 30, 2014 Portugal sub-sovereign debt [Member] EUR (€)	Jun. 30, 2014 Hungarian-infrastructure bonds [Member] USD ($)	Jun. 30, 2014 Spanish and Hungarian Debt [Member] USD ($)	Jun. 30, 2014 Spanish and Hungarian Debt [Member] USD ($)	Jun. 30, 2014 Manufactured Housing Loans [Member] USD ($)	Jun. 30, 2014 Manufactured Housing Loans [Member] USD ($)	Jun. 30, 2014 Below investment grade (BIG) USD ($)	Dec. 31, 2013 Below investment grade (BIG) USD ($)	Jun. 30, 2014 Below investment grade (BIG) Life Insurance [Member] USD ($)	Jun. 30, 2014 Below investment grade (BIG) Prime first lien USD ($)	Dec. 31, 2013 Below investment grade (BIG) Prime first lien USD ($)	Jun. 30, 2014 Below investment grade (BIG) Alt-A first lien USD ($)	Dec. 31, 2013 Below investment grade (BIG) Alt-A first lien USD ($)	Jun. 30, 2014 Below investment grade (BIG) Option ARM USD ($)	Dec. 31, 2013 Below investment grade (BIG) Option ARM USD ($)	Jun. 30, 2014 Below investment grade (BIG) First mortgage, subprime USD ($)	Dec. 31, 2013 Below investment grade (BIG) First mortgage, subprime USD ($)	Jun. 30, 2014 Below investment grade (BIG) HELOCs USD ($)	Dec. 31, 2013 Below investment grade (BIG) HELOCs USD ($)	Jun. 30, 2014 Below investment grade (BIG) Asset-backed Student Loan [Member] USD ($)	Jun. 30, 2014 Below investment grade (BIG) Public Finance Student Loan [Member] USD ($)	Jun. 30, 2014 Below investment grade (BIG) Trust Preferred Securities Collateralized Debt Obligations USD ($)	Jun. 30, 2014 Below investment grade (BIG) Manufactured Housing Loans [Member] USD ($)	Jun. 30, 2014 Below investment grade (BIG) Ballantyne Re Plc and Orkney Re I I Plc [Member] Life Insurance [Member] Transaction	Jun. 30, 2014 Sewer [Member] Public Finance City of Detroit [Member] USD ($)	Jun. 30, 2014 Water Sector [Member] Public Finance City of Detroit [Member] USD ($)	Jun. 30, 2014 Municipal Bonds State General Obligation [Member] Public Finance City of Detroit [Member] USD ($)	Jun. 30, 2014 City Certificates of Participation [Member] Public Finance City of Detroit [Member] USD ($)	Jun. 30, 2014 Refinancing Risk on Infrastructure Transactions [Member] USD ($) Transaction	Jun. 30, 2014 Refinancing Risk on Infrastructure Transactions [Member] Minimum	Jun. 30, 2014 Refinancing Risk on Infrastructure Transactions [Member] Maximum	Jun. 30, 2014 Puerto Rico USD ($)	Jun. 30, 2014 Puerto Rico Below investment grade (BIG) USD ($)	Jun. 30, 2014 Puerto Rico Below investment grade (BIG) USD ($)	Aug. 07, 2014 Subsequent Event [Member] Sewer and Water [Member] Public Finance City of Detroit [Member] USD ($)
Guarantor Obligations [Line Items]
Financial Guaranty Insurance Contracts Expected Future Recoveries for Breached Representations and Warranties Amount of New Transactions	$ 0		$ 6,000,000	$ 0		$ 6,000,000
Net par outstanding amount	5,780,000,000			5,780,000,000																																																			2,700,000,000	2,700,000,000	1,900,000,000	800,000,000	4,700,000,000													239,000,000	239,000,000			2,700,000,000											201,000,000	248,000,000	1,600,000,000	169,000,000		1,000,000,000	784,000,000	128,000,000	175,000,000	3,100,000,000						5,200,000,000
Recovery Percentage																																																												74.00%
Estimated Years for Recoveries on Infrastructure Transactions																																																																																																	10 years	35 years
Financial Guaranty Insurance Contracts Principal Amount, Percent in Credit Default Swap Form																																																											71.00%
Number of Risks as of the end of the period with R&W Benefit Recorded (in policies)	44	[1]		44	[1]		42	[1]			1	[1]		1	[1]		1	[1]	23	[1]		23	[1]		19	[1]	10	[1]		10	[1]		9	[1]	4	[1]		4	[1]		5	[1]					2	[1]		2	[1]		4	[1]
Outstanding Principal and Interest on Representations and Warranties Policies	4,214,000,000			4,214,000,000			5,011,000,000				35,000,000			35,000,000			38,000,000		2,739,000,000			2,739,000,000			2,856,000,000		553,000,000			553,000,000			641,000,000		668,000,000			668,000,000			998,000,000						70,000,000			70,000,000			320,000,000
Change in recovery assumptions as the result of recovery success	17,000,000		6,000,000	27,000,000		17,000,000
Estimated increase (decrease) in defaults that will result in additional (lower) breaches	(11,000,000)		(4,000,000)	(11,000,000)		(3,000,000)
Settlements and anticipated settlements	10,000,000		38,000,000	45,000,000		180,000,000
Accretion of discount on balance	3,000,000		5,000,000	6,000,000		8,000,000
Total	19,000,000		51,000,000	67,000,000		208,000,000					0		0	(1,000,000)		0			(2,000,000)		(5,000,000)	1,000,000		(13,000,000)			11,000,000		13,000,000	20,000,000		166,000,000			1,000,000		(5,000,000)	29,000,000		(1,000,000)			0	(3,000,000)	(3,000,000)	(12,000,000)	9,000,000		51,000,000	21,000,000		68,000,000
Loss and loss adjustment expense reserve	775,000,000			775,000,000			592,000,000																																																																					598,000,000
Number of Transactions Insured																																																																																											2
Projected Expected Loss to be Paid																																																							73,000,000	73,000,000			32,000,000				339,000,000	339,000,000	281,000,000	264,000,000										87,000,000
Net Expected Loss Development After Recoveries for R&W	23,000,000		87,000,000	35,000,000		(11,000,000)			3,000,000	16,000,000	(7,000,000)		7,000,000	(10,000,000)		13,000,000			4,000,000		(7,000,000)	12,000,000		2,000,000			(24,000,000)		21,000,000	(39,000,000)		(117,000,000)			6,000,000		23,000,000	(1,000,000)		48,000,000							(33,000,000)		(31,000,000)	(31,000,000)		(34,000,000)			5,000,000	8,000,000			19,000,000				82,000,000	105,000,000						1,000,000	1,000,000	1,000,000	1,000,000
Financial Guaranty Insurance Contracts Principal Amount Gross																																																																			435,000,000	88,000,000	618,000,000
Liability for Unpaid Claims and Claims Adjustment Expense Net Largest Single Loss																																																							32,000,000	32,000,000																26,000,000	26,000,000
Insured Financial Obligations Outstanding Principal Amount Excluding Loss Mitigation Bonds	437,576,000,000			437,576,000,000			459,107,000,000				509,000,000			509,000,000			541,000,000		3,369,000,000			3,369,000,000			3,590,000,000		833,000,000			833,000,000			937,000,000		5,736,000,000			5,736,000,000			6,130,000,000						1,901,000,000			1,901,000,000			2,279,000,000																					21,575,000,000	22,538,000,000		381,000,000	403,000,000	2,621,000,000	2,728,000,000	548,000,000	598,000,000	1,824,000,000	1,945,000,000	1,671,000,000	1,897,000,000													5,200,000,000	4,954,000,000	5,000,000,000
Financial Guaranty Insurance Contracts Principal Amount Net																																																												2,100,000,000	119,000,000	337,000,000					312,000,000	77,000,000	583,000,000
Expected LAE for mitigating claim liabilities	33,000,000			33,000,000			34,000,000																																																															50,000,000	50,000,000
Number of Infrastructure Transactions																																																																																																2
Financial Insurance Guaranty, Infrastructure Finance, Potential Liability																																																																																																$ 1,800,000,000

[1] A risk represents the aggregate of the financial guaranty policies that share the same revenue source for purposes of making Debt Service payments. This table shows the full future Debt Service (not just the amount of Debt Service expected to be reimbursed) for risks with projected future R&W benefit, whether pursuant to an agreement or not.